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We present observations from an exhumed subduction complex that resembles the environment of modern deep episodic tremor and slow slip (ETS). We focus on the Cycladic Blueschist Unit on Syros Island in Greece. Syros metabasites consist of blueschists and eclogites that record prograde deformation, with peak metamorphism of 1200-1600 MPa and 450-550 °C. Field observations reveal that coexistence of blueschist and eclogite sets up an important rheological contrast: blueschists show distributed viscous dislocation creep, whereas eclogites dispersed within the blueschist matrix show brittle shear fractures and veins. These observations are consistent with the inferred prominent role of high fluid pressures from geophysical studies, but are inconsistent with models of deep ETS that invoke changes in rate-and-state friction parameters along a narrow fault. Instead, we suggest deep ETS may be controlled by coupled brittle-viscous deformation in partially eclogitized oceanic crust embedded within high-fluid-pressure patches along the plate interface.

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[1] Burial histories of subduction zone rocks are often difficult to accurately constrain, owing to a lack of robust mineral geobarometers applicable to high pressure mineral assemblages. Knowledge of the depthhistories of subduction is, however, required for our understanding of global geochemical cycles, subduction-related seismicity, and the evolution of destructive tectonic boundaries. The high spatial resolution of quartz inclusion geobarometry can be used to determine pressure evolution during metamorphic growth of individual garnet crystals. Quartz inclusions in garnet from Sifnos, Greece, preserve such a record of the pressure of garnet growth, allowing detailed reconstruction of the metamorphic evolution of these rocks. Pressure-dependent Raman spectra of quartz inclusions were combined with elastic modeling to infer the conditions at which they were trapped during garnet growth. All measured inclusions suggest that garnet growth occurred between 19 and 20.5 kbars, with little evidence for significant pressure variation during the garnet growth interval, which is interpreted to record $100 C of heating. Coupled with thermometry and geochronology, these results show that early, cold burial was followed by a phase of rapid heating, which immediately preceded exhumation. Garnet growth occurred primarily during this heating phase.Components: 6,458 words, 5 figures, 1 table.

The Northern Highlands Terrane of Scotland hosts several thrust nappes that were deformed and metamorphosed during the Silurian Scandian orogeny. Quantitative petrological analysis of metamorphic assemblages indicates that the hinterland-positioned Naver nappe experienced decompression heating from 8–9 kbar and 600°C to 6–7 kbar and 700°C. Monazite–xenotime thermometry and geochronology delineate a detailed temperature–time history for the Naver nappe. Monazite often exhibits compositional zoning, which is used to establish multiple temperature–time points in several samples. These data indicate that the Naver nappe experienced relatively fast heating ( c. 50°C myr −1 ) and relatively slow cooling (15–20°C myr −1 ), with peak temperatures occurring at c. 425 Ma. This temperature–time evolution is compatible with the early Emsian (407–403 Ma) deposition of unmetamorphosed conglomerates that rest on high-grade metamorphic rocks in the Naver nappe, but requires an acceleration in the cooling rate to 40–50°C myr −1 at 420–410 Ma. Geochronological constraints from this study and previous work suggest that deformation and metamorphism in the hinterland of the Scandian orogen in northern mainland Scotland are younger than the c. 430 Ma deformation in the foreland-positioned Moine thrust zone. We postulate that heat from pervasive granitic intrusions in the Naver nappe weakened the crust, allowing deformation to retreat to the hinterland of the orogen. Supplementary material: A description of our analytical methods, all U–Pb-trace element data, additional figures explaining our petrological analysis and other relevant data are available at: https://doi.org/10.6084/m9.figshare.c.4458041